Definition: Meta-information (information about information) that is associated with a web page and placed in the HTML but not displayed on the page for the user to see. There are a range of meta tags, only a few of which are relevant to search engine spiders. Two of the most well-known meta tags are the meta description and meta keywords; unfortunately these are ignored by most major search engines, including Google.
FirstClickSEO Commentary: These are invisible tags that help the search engines index your site.